Why does a guy sitting on a solo seat suspended by torsion springs, on a rigid frame, not get donkey kicked over the handlebars?  Those springs aren't dampened.  I've seen guys riding rigids defnitely get bounced around, and it's a rougher ride, but I've never seen them pogo sticking like those videos.  What's the difference?

(need to add a disclaimer after I read this...not trying to sound argumenative & defensive.  This was a honest question.)

I've ridden a Power Plus, it isn't a very functional set up. It will damp some of the road surface irregularities, but you're going to notice them.

Find a swingarm off a chain drive Yammerscammer Very Slow (Virago). It used a very British shock mounting, single coil over that connect to the frame backbone. Copy the upper mount on your Savage frame and graft on the swingarm, paint it black to match the rest of the frame cradle.

Find a wrecked VN800, style doesn't matter, and get the rear section of the frame with swingarm and shock. Chop off the back of your frame and graft on the Kawasaki parts. Use the Kawi rear wheel, sprocket and convert your bike to chain drive.

XV920 Yamaha...

VN800 Kawasaki...

The Yammie has a little bit more obvious suspension, the VN has a cleaner look.

And the best Japanese middle weight cruisers ever made were the VN800A, VN800 Drifter and VN800B (Classic). The 800A with a bit of tinkering will break the drive unit in its 140 mph speedometer as you go sailing past "pegging" the needle... or the cable (1st time) or rear drive unit (2nd time) I did that with my old one.